Andean Geology 45 (2): 249-273. May, 2018 Andean Geology doi: 10.5027/andgeoV45n2-3049 www.andeangeology.cl

The structure of the northern Agrio fold and thrust belt (37°30’ S), Neuquén Basin, Argentina

*Fernando Lebinson1, Martín Turienzo1, Natalia Sánchez1, Vanesa Araujo1, María Celeste D’Annunzio1, Luis Dimieri1

1 Instituto Geológico del Sur (INGEOSUR), Universidad Nacional del Sur-CONICET, San Juan 670, 8000 Bahía Blanca, Argentina. [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]

* Corresponding author: [email protected]

ABSTRACT. The Agrio fold and thrust belt is a thick-skinned orogenic belt developed since Late Cretaceous in response to the convergence between the Nazca and South American plates. The integration of new structural field data and seismic line interpretation allowed us to create two balanced cross-sections, which help to analyse the geometry of both thick and thin-skinned structures, to calculate the tectonic shortenings and finally to discuss the main mechanisms that produced this fold and thrust belt. The predominantly NNW-SSE structures show varying wavelengths, and can be classified into kilometer-scale first order basement involved structures and smaller second, third and fourth order fault-related folds in cover rocks with shallower detachments. Thick-skinned structures comprise fault-bend folds moving into the sedimentary cover, mainly along Late Jurassic evaporites, which form basement wedges that transfer the deformation to the foreland. Thus, shortenings in both basement and cover rocks must be similar and consequently, by measuring the contraction accounted for thin-skinned structures, is possible to propose a suitable model for the thick skinned deformation. The balanced cross-sections indicate shortenings of 11.2 km (18%) for the northern section and 10.9 km (17.3%) for the southern section. These values are different from the shortenings established by previous works in the region, reflecting differences in the assumed model to explain the basement-involved structures. According to our interpretation, the structural evolution of this fold and thrust belt was controlled by major basement-involved thrust systems with subordinate influence of inversion along pre-existing normal faults during the Andean compression.

Keywords: Thick and thin-skinned structures, Balanced cross-sections, Thrust systems, Basement involvement, Andean shortenings.

RESUMEN. La estructura del sector norte de la faja plegada y corrida del Agrio (37°30’ S), Cuenca Neuquina, Argentina. La faja corrida y plegada del Agrio es un cinturón orogénico de piel gruesa desarrollada desde el Cretácico Tardío en respuesta a la convergencia entre las placas de Nazca y Sudamérica. La integración de nuevos datos estructurales de campo e interpretación de líneas sísmicas permitió construir dos secciones balanceadas y, de esta manera, poder analizar la geometría de las estructuras del tipo piel gruesa y piel fina, calcular los acortamientos tectónicos y finalmente discutir los principales mecanismos que formaron esta faja corrida y plegada. Las estructuras predominantemente NNO- SSE muestran diferentes longitudes de onda, y pueden ser clasificadas en estructuras kilométricas de primer orden que involucran al basamento y en pliegues más pequeños de segundo, tercer y cuarto orden en la cubierta sedimentaria relacionados con fallas con despegues menos profundos. Las estructuras de piel gruesa comprenden pliegues por flexión de falla que se insertan en la cubierta sedimentaria, principalmente a lo largo de las evaporitas del Jurásico Tardío, y forman cuñas de basamento que transmiten la deformación al antepaís. En consecuencia, los acortamientos del basamento y de la cubierta deben ser similares y, por consiguiente, al medir la contracción de las estructuras de piel delgada, es posible proponer un modelo adecuado para la deformación de piel gruesa. Las secciones balanceadas indican acortamientos de 11,2 km (18%) para la sección norte y 10,9 km (17,3%) para la sección sur. Estos valores son diferentes a aquellos 250 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina previamente obtenidos en trabajos anteriores en la región, y reflejan principalmente las diferencias en los supuestos del modelo considerado para explicar las estructuras que involucran al basamento. De acuerdo a nuestra interpretación, la evolución estructural en este cinturón corrido y plegado estuvo controlada principalmente por sistemas de corrimientos que involucran al basamento, con una influencia subordinada de la inversión de fallas normales preexistentes durante la compresión andina.

Palabras clave: Estructuras de piel gruesa y piel fina, Secciones balanceadas, Sistemas de corrimientos, Participación del basamento, Acortamientos andinos.

1. Introduction Two main structural models have been used in order to explain the first order basement involved The Agrio fold and thrust belt (Ramos, 1978) is structures of the fold and thrust belts in the Neuquén part of the Southern Central Andes and it is located Basin. It has been proposed that most of the Andean at the Central-Western region of Argentina (Fig. 1a). contractional deformation took place by the tectonic The contractional deformation in this sector began inversion of pre-existing normal faults, inherited in the Late Cretaceous related to the convergence from the Late Triassic-Early Jurassic rifting stage of between the Nazca plate and the South American plate. the Neuquén Basin (Vergani et al., 1995; Booth and The region of study is located in the Neuquén Basin, Coward, 1996; Ramos, 1998; Zapata et al., 1999; which is one of the most important oil producing Zapata and Folguera, 2005; Zamora Valcarce, 2007; sedimentary basins of Argentina. In this zone, Rojas Vera et al., 2015). Other studies consider that several works on the stratigraphy and the structure basement-involved thrust systems, formed by low have been conducted because of the scientific and angle reverse faults produced during the Andean economic interest due to the presence of hydrocarbon compression, are the most suitable structures in deposits (Groeber, 1929, 1946; Zöllner and Amos, order to produce the observed thick and thin-skinned 1973; Minniti et al., 19861; Gulisano and Gutiérrez deformation (Minniti et al., 1986; Nocioni, 1996; Pleimling, 1995; Vergani et al., 1995; Kozlowski et Kozlowski et al., 1998; Turienzo et al., 2014, 2018; al., 1998; Zapata et al., 1999; Zapata and Folguera, Sánchez et al., 2015; Lebinson et al., 2015a). 2005; Leanza et al., 2006; Zamora Valcarce et al., The study area is situated between Chos Malal 2009; Rojas Vera et al., 2015; Turienzo et al., 2014, city and Taquimilán town, to the west of the Neuquén 2018; Sánchez et al., 2014, 2015). Province, Argentina (Fig. 1c). The purpose of this The Agrio fold and thrust belt (FTB) was initially work is to characterize the structural style of the defined by Bracaccini (1970) as “fosa del Agrio” northern segment of the Agrio FTB based on a and is located between Chos Malal and Las Lajas detailed field mapping of the exposed structures. cities (Fig. 1b). According to the predominance of Two balanced structural cross-sections, supported thick or thin-skinned structures, the Agrio FTB has by seismic data, allow the analysis of the geometry been divided into an inner and outer zone (Ramos, of the major structures at depth. Additionally, the 1998). The inner zone, at the west, is characterized kinematics of complex thin-skinned structures was by basement-involved anticlines and synclines with reconstructed using the Fault-FoldForward software large wavelengths. The eastern outer zone is dominated (Allmendinger, 20123), in order to visualize its by fault-related folds with smaller wavelengths that sequential evolution. Restoration of the structural involve the Mesozoic sedimentary sequence above cross-sections allows to determine the tectonic a decollement level in the Late Jurassic evaporites shortenings. A comparison of these shortenings (Viñes, 19852; Zapata et al., 2002). Thick-skinned with those previously determined in surrounding structures have also been recognized in the outer areas help to discuss the influence, or lack of it, in zone from seismic subsurface information (Zapata the different structural models proposed for this et al., 2002; Zamora Valcarce et al., 2009). sector of the Andes.

1 Minniti, S.; Gutiérrez Pleimling, A.; Artega, M.; Pestalardo, F. 1986. Análisis estructural y estratigráfico de la Faja Plegada Neuquina a la latitud de 37°30’, Departamentos de Ñorquin y Pehuenches, provincia de Neuquén (Inédito). Informe Gerencia General de Exploración. Yacimientos Petrolíferos Fiscales, Subgerencia de Geología, Comisión Geológica No. 1: 64 p. 2 Viñes, R.F. 1985. Estilos estructurales en la faja occidental neuquina (Inédito). Yacimientos Petrolíferos Fiscales S.A: 6 p. 3 Allmendinger, R.W. 2012. FaultFoldForward.v.6. http://www.geo.cornell.edu/geology/faculty/RWA/programs/faultfoldforward-v-6.html. (last visit 16/03/2017). Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 251

FIG. 1. a. Present-day tectonic setting at the convergent border between the Nazca and South American plates; b. Limits of the Mesozoic Neuquén Basin with the location of the Agrio, Chos Malal and Malargüe fold and thrust belts (FTB); c. Digital elevation model of the study area in the northwest of the Neuquén Province.

2. Geological framework during the Permian-Early Triassic produced the Choiyoi mesosilicic and silicic magmatic province The Andes of Neuquén (36°-38° S latitude) have (Llambías et al., 2007). According to Llambías and distinctive characteristics due to the alternation Sato (2011), this magmatism was favoured by the of extensional and compressional tectonics, with extensional collapse of the San Rafael orogen (Fig. 2). important changes in its associated magmatism, Permian granitoids of the Choiyoi Group are exposed developed since Late Paleozoic to Recent (Ramos, in the Cordillera del Viento (Fig. 1c), to the north of 1999; Ramos and Folguera, 2005). The development the study area, intruding Carboniferous strata of the of an active margin at the western border of Gondwana Andacollo Group (Zöllner and Amos, 1973; Danieli et 252 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina

FIG. 2. Tectonostratigraphic chart of the Neuquén Basin in the study region (based on Gulisano and Gutiérrez Pleimling, 1995; Leanza et al., 2006, 2013). al., 2011). The Late Triassic-Early Jurassic generalized 1995, Arregui et al., 2011). This basin has a triangular extension is related with the breakup of Gondwana, shape, limited to the northeast by the San Rafael that produced NW-trending half-grabens filled by block and to the southeast by the North Patagonian volcaniclastic synrift sequences of the Precuyano Cycle Massif (Fig. 1b), and to the west it was connected (Vergani et al., 1995; Franzese and Spalletti, 2001; with the Pacific Ocean through the volcanic arc. The Carbone et al., 2011). From the Early Jurassic to the nature of the sedimentary infill during this period Early Cretaceous, subduction at the western margin was controlled by a complex combination between of Gondwana caused a regional phase of back-arc tectonic, eustatic and climatic factors, which gave rise thermal subsidence that favoured the accumulation to transgressive-regressive marine cycles alternating of ~7,000 m of marine, continental and volcaniclastic with periods of continental sedimentation (Vergani rocks in the Neuquén Basin (Digregorio and Uliana, et al., 1995; Legarreta and Uliana, 1996, Arregui et 1980; Legarreta and Uliana, 1996; Vergani et al., al., 2011). The Cuyo, Lotena, Mendoza and Bajada Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 253 del Agrio groups were deposited during this stage of of synclines to the south of Chos Malal (Fig. 3). thermal subsidence (Fig. 2). Marine black shales of An important retroarc volcanism took place in the Cuyo Group (Los Molles Formation) are the oldest the foreland region during the Neogene, that are rocks exposed in the northern Agrio FTB (Fig. 3). evidenced by basalts, andesites and tuffs (Ramos The overlying Lotena Group contains Late Jurassic and Barbieri, 1989; Kay et al., 2006; Dyhr et evaporites (Auquilco Formation), which constitute al., 2013), that in the study area form the Cerro one of the most important decollement horizons in Villegas (Fig. 3). Finally, basalts covering the Loma the Agrio FTB (Zapata et al., 1999, 2002; Zamora Tilhué correspond to lava flows coming from the Valcarce et al., 2006). The Mendoza Group includes Plio-Quaternary Tromen-Tilhué volcanic complex the two most important hydrocarbon source rocks of (Llambías et al., 2011). the Neuquén Basin (Vaca Muerta and Agrio formations, Fig. 2). The Agrio Formation also contains one of 3. Structure the main reservoir units, the Avilé Member, formed by fluvial-aeolian sandstones that reflect a relatively Numerous NNW-SSE trending structures, with major sea-level drop (Veiga et al., 2002). variable scale and vergence, are notably well exposed Sandstones of the Lower Troncoso Member in the northern segment of the Agrio FTB. In several (Huitrin Formation), in the Bajada del Agrio Group field trips, around 500 GPS points were collected, each (Fig. 2), are the best hydrocarbon reservoir rocks one including geological descriptions and structural in this sector of the basin. Shallow marine rocks in data. This field information together with georeferenced this group represent the last marine connection of Landsat and Google Earth satellite imagery allowed the Neuquén Basin with the Pacific Ocean and the us to elaborate a detailed geological map (Fig. 3), subsequent deposition of continental sediments was which comprises an area of 1,742 km2 (67x26 km). probably influenced by the growth of the magmatic Different orders of folds were distinguished based arc (Legarreta and Uliana, 1991). on their wavelength and the involved units, as it Since the early Late Cretaceous, the Neuquén was done in the Chos Malal fold and thrust belt basin evolved into a contractional retroarc foreland (Kozlowski et al., 1996; Turienzo et al., 2014, 2018; basin due to the initial development of the fold Sánchez et al., 2015). The thick skinned structures and thrust belt, probably related to a combination form first order folds, which involve basement rocks of an increasing convergence rate (Cobbold and and wavelengths of around 8-12 km. The thin-skinned Rossello, 2003) and a shallowing of the Benioff structures correspond to fault-related folds with shorter zone (Ramos, 1998; Ramos and Folguera, 2005). A wavelengths that can be classified in second, third continental sedimentary environment prevailed in and fourth order depending on the detachment level the Late Cretaceous, with the syntectonic deposition within the stratigraphic pile. Based on the mapped of the Neuquén Group (Tunik et al., 2010). The structures and with the aid of 2D seismic profiles, continuity of the flexural load allowed a marine two E-W balanced cross-sections were constructed transgression from the Atlantic Ocean represented in order to characterize the structural style of this by the shallow marine rocks of the Malargüe Group segment of the Agrio FTB. (Fig. 2). Widespread igneous rocks were derived from a subduction related magmatic arc, including 3.1. Structures of the north sector: cross-section a-a’ the Late Cretaceous to Eocene Naunauco Group (Zamora Valcarce, 2007). Subvolcanic rocks in The reconstruction of the tectonic structures in the Cerro Caicayén (Fig. 3), with 40K/40Ar age of depth along the north cross-section was analysed 44.7±2.2 Ma (Llambías and Rapela, 1989), and the in conjunction with the information provided by Cerro Naunauco, with 40Ar/39Ar age of 65.5±0.46 Ma seismic line 15009 (Fig. 4). Despite it´s poor quality, (Zamora Valcarce et al., 2006), were intruded this line shows west-dipping reflectors that allows during this stage. Synorogenic foreland sequences the interpretation of the basement-cover contact related to Neogene contraction in the Agrio FTB geometry. The elevation of the basement-cover accumulated in piggyback basins (Ramos, 1998; contact at the eastern sector of the seismic section is Repol et al., 2002; Zapata et al., 2002; Cervera and linked to the west-dipping Las Yeseras thrust (Fig. 4), Leanza, 2009), are scarcely preserved in the core responsible of a notable basement-cored anticline 254 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina Tilhué 13. Tilhué Taquimilán anticline; 12. Taquimilán Truquico anticline; Truquico 11. Truquico syncline; 10. Truquico 2. El Cholar anticline; 3. Manzano 4. Rahueco synclinorium; 5. Tres Chorros anticline; Taquimilán Centro anticline; Taquimilán 9. Las Yeseras anticline; 20. Cerro Yeseras 16. Maitenes anticline; 17. Río Neuquén 18. San Eduardo 19. Las Rahueco anticline; 8. Naunauco syncline; Pampa Tilhué anticline; Tilhué 15. Pampa 7. Cerro Pitrén anticline; Tilhué Este anticline; Tilhué 14. Tralalhué syncline; 6. Tralalhué Oeste anticline; Villegas syncline. Villegas 3. Geological map of the northern segment of the Agrio FTB. 1. 3. Geological map of the northern segment FIG. Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 255

FIG. 4. Interpreted and partially interpreted two-way travel time seismic section 15009. Noticeable basement uplift at the eastern zone occurred in the hangingwall of the Las Yeseras thrust. The amplified zones depict areas in the seismic with unconfomities and varying thickness interpreted as synrift deposits related to normal faults, with scarce evidence of significant inversion. exposed outside the studied area. Although there are The westernmost structure in the north sector no exploration wells along this seismic line, these of the study area is the Rahueco anticline, a doubly strong and continuous reflectors can be assigned to plunging fold with an approximate wavelength of the Jurassic sedimentary sequences. Below them, 12 km (Figs. 3 and 6). The Early Jurassic black shales diffuse reflectors with varying thicknesses suggest of the Los Molles Formation, placed in the core of the occurrence of synrift deposits of the Precuyano this anticline, correspond to the oldest rocks exposed Cycle (Fig. 4). These synrift sequences have been in the Agrio FTB region (Fig. 6 a and b). Measured related with high angle normal faults, which have dips of around 15º-25º W in the backlimb and 40º- been slightly inverted during the subsequent 60º E in the forelimb, which increase up to 70º-80º E in compressive deformation in the Andes. Bed-length beds of the Mulichinco and Agrio formations (Fig. 6a), restoration of cross-section a-a’ (Fig. 5) allows to demonstrate the eastward-vergence of the Rahueco establish 11.2 km (18%) of shortening. anticline. The steepening of the eastern limb is 256 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina

FIG. 5. Interpreted and restored structural cross-section a-a’. Major basement-involved thrusts produce first order thick-skinned wedges, which move eastward over the Auquilco Formation (Lotena Group) creating second and third order fault-related folds in the sedimentary cover. 1. Rahueco anticline; 4. Rahueco synclinorium; 7. Cerro Pitrén anticline; 9. Taquimilán Centro anticline; 11. Truquico anticline; 13. Tilhué Oeste anticline; 14. Tilhué Este anticline; 15. Pampa Tilhué anticline.

FIG. 6. a. Photograph showing the eastward-steepening of the Mendoza Group beds in the forelimb of the Rahueco anticline; b. Southward view of the backthrust and the associated west vergent fold involving the Cuyo Group in the forelimb of the Rahueco anticline. Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 257 related to a set of minor backthrusts in the front Rahueco syncline, where a small backthrust generated of this major first order structure. One of these a repetition of the Huitrín Formation (Fig. 7a). west-directed thrusts produced a tight asymmetric One of the most important thin-skinned folds in anticline involving the Tábanos Formation (Fig. 6b). the study area is the Cerro Pitrén anticline (Cervera Igneous rocks that form the Cerro Caicayén are mainly and Leanza, 2009), a doubly-plunging fold cored concentrated in the core of the Rahueco anticline, by the Mulichinco Formation with a wavelength and were intruded using the backthrusts as feed of approximately 6 km (Fig. 3). The western limb channels (Lebinson et al., 2015b). Based on field of the Cerro Pitrén anticline has variable dips (30°- data and the geometry of the structure, the Rahueco 50° W), while the eastern limb is steep, or even anticline has been interpreted as a basement wedge overturned, defining a clear east vergent structure. related to a major west-dipping thrust (Fig. 5). This The development of the Cerro Pitrén anticline is tectonic wedge corresponds to a first order structure interpreted as a second-order fault-bend fold, with a that develops as a fault-bend anticline moving lower detachment in the Auquilco Formation and an along the evaporites of the Auquilco Formation and upper decollement on top of the Vaca Muerta shales transferring shortening to the east. Thus, the thin- (Fig. 5). Thus, there is a transference of displacement skinned structures in the area that involve the rocks to the east over the upper decollement, which allows of Tordillo Formation and overlying units developed to generate third order folds involving the Mulichinco above this detachment level (Fig. 5). and Agrio formations (Taquimilán Centro and Truquico The Rahueco synclinorium, located to the anticlines). Subsequently, the forelimb of the Cerro east of the Cerro Caicayén, is cored by gently Pitrén anticline is cut by a breakthrough thrust that folded volcanic rocks (of uncertain age) that cover causes an increase in its dip. A comparable model of unconformably the folded Mesozoic rocks (Figs. 3 structural evolution has been applied in the Chos Malal y 7). A complex deformation occurs at the eastern FTB (Turienzo et al., 2014; Sánchez et al., 2015). limb of this synclinorium. Rocks of the Agrio, The Taquimilán Centro anticline, located to the east Huitrín and Rayoso formations form a tight west- of the Cerro Pitrén, has a wavelength of approximately vergent anticline of approximately 1 km wavelength 3.5 km and N-S trend (Fig. 3). The west limb dips (Figs. 7a). Two minor backthrusts deform the Avilé 42° W while the eastern one dips 75° E, thus the Member in the core of this anticline, while in its vergence of the fold is to the east. An N-S thrust backlimb, the Pitrén backthrust, displaces and rides the Cerro Pitrén anticline over the backlimb repeats the Huitrín Formation (Fig. 7b). The average of the Taquimilan Centro anticline, juxtaposing the orientation of the fault plane is 092°/54° (dip direction/ Mulichinco Formation on top of the Agua de la Mula dip) with fault-striae with pitch values ranging from Member (Agrio Formation). The morphology of the 5° to 60° S and steps indicating a component of Taquimilán Centro anticline allows to interpret it as sinistral shearing for the Pitrén backthrust. To the a fault propagation fold detached above the shales north, near to Route 6, there is another west-verging of the Vaca Muerta Formation (Fig. 5). anticline whose backlimb and forelimb dip 45º NE The Truquico syncline, located to the south of Chos and 64º SW respectively (Fig. 7a). In the core of Malal, is a broad NNW trending syncline cored by this anticline, small thrusts repeat the Agua de la Cenozoic rocks (Fig. 3). The Miocene synorogenic Mula Formation stratigraphy (Fig. 7c). Based on strata of the Chos Malal Formation lie unconformably Mitra (2002), these accommodation structures were over the Rayoso Formation and they are, in turns, classified as intoanticline thrusts. The reconstruction covered by a thin horizontal sequence assigned to of the two west-vergent anticlines in the Rahueco Rincón Bayo and Pampa Encima formations (Cervera synclinorium allows to relate them with shallow and Leanza, 2009). The Truquico syncline is located backthrusts detached on the lower section of the Agrio into a triangle zone formed between the east-vergent Formation and classified as third order structures. Cerro Pitrén anticline and the west-vergent Truquico Occasionally the faults associated with these third anticline (Fig. 5). The Truquico anticline, which order folds reach a shallower detachment level, as the is exposed at the intersection of Routes 40 and 6, shales of the upper portion of the Agrio Formation, has a marked west vergence and a wavelength of creating smaller-scale fourth order structures. An approximately 4 km (Fig. 3). The forelimb dips 80°- example of this occurs at the southern hinge of the 85° W, and the uppermost beds of the Agrio, Huitrín 258 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina

FIG. 7. a. Google Earth image and detailed geological map between Tres Chorros and Cerro Pitrén; b. Photograph of the Pitrén backthrust, a transpressive fault that duplicates the Huitrín Formation; c. Southward view of a third-order anticline, containing minor faults in the core interpreted as into-anticline thrusts. and Rayoso formations are overturned (Fig. 8a). The A complex structural configuration combining backlimb of the Truquico anticline has a constant dip several NNW folds and thrusts occurs at the Loma of 20° E, measured in the Avilé Member along the Tilhué area, to the east of Chos Malal (Fig. 8a). road-cut outcrop, in a classical stop to visualise this Three anticlines involving the Mulichinco Formation, unit in this part of the Neuquén Basin. East of the separated by two synclines that contain shales of the Neuquén River, the Avilé Member is tightly folded Pilmatué Member, dominate the structure of this forming a NNW syncline that is cored by evaporites area. These folds have wavelengths in the order of of the Huitrín Formation, a structure that can be well 3-4 km. The Tilhué Oeste anticline is located on the observed along the access road to Chos Malal (Figs. 3 western slope of the Loma Tilhué outcrop and has and 8a). Based on the dimensions of the fold and the a west asymmetry, as evidenced by its overturned involved units, the Truquico anticline is interpreted as forelimb in the Avilé Member. The Tilhué Este a third order fault-propagation fold detached above anticline can be observed along the Neuquén River shales of the Vaca Muerta Formation (Fig. 5). where shales of the Vaca Muerta Formation crop out Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 259

FIG. 8. a. Detailed geological map showing the structures in the Loma Tilhué area. b. Photograph of the forelimb of the Pampa Tilhué anticline and the frontal syncline cored by the Avilé Member (see location in figure 3). in its core. In the central area of the hill, this fold 43° to 82° NE in the forelimb and 56° SW in the is cut by a west-dipping thrust that juxtaposes the backlimb, while toward the south the fold-vergence Vaca Muerta Formation with the Agrio Formation changes (Fig. 8b). The sequence of deformation in (Fig. 8a). Finally, the Pampa Tilhué anticline shows this complex area will be analysed in the section 4 locally an eastward vergence, with dips varying from of this paper. 260 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina

3.2. Structures of the south sector: cross-section b-b’ folded forming the El Cholar and Manzano anticlines (Fig. 3). These folds have wavelengths between The structural cross-section b-b’ is parallel to 7 and 9 km and they locate south of the Rahueco seismic line 15007, whose poor quality does not allow anticline, thus basement rocks should be involved an accurate subsurface interpretation (Fig. 9). The at depth. Both anticlines have east vergence, in most noticeable feature in this line is the shallowing which their eastern limbs dips 50°-60° E (Fig. 10a), of the basement toward the east, related to the Las and their western limbs dip of 15°-30° W (Fig. 10b). Yeseras thrust as can be seen in seismic line 15009. The El Cholar anticline is interpreted as a thick- In addition, some disturbed areas affecting shallow skinned fault-propagation fold, with relatively reflectors correlate with the exposed anticlines, minor displacement, and the Manzano anticline as a reconstructed as second order fault-related folds. basement-involved fault-bend fold whose eastward Shortening resulting from the bed-length restoration movement produce thin-skinned deformation in the of this cross-section is 10.9 km (17.3%), slightly less adjacent cover rocks (Fig. 9). Minor-scale folding than in the north structural cross-section (Fig. 9). affects the Tordillo Formation in the proximities The oldest rocks exposed in this region correspond of Tres Chorros (Figs. 3 and 10c), which can to the Late Jurassic Tordillo Formation, which are be considered thin-skinned structures probably

FIG. 9. Interpreted and restored structural cross-section b-b’. Because of the poor quality of seismic line 15007, it was not possible to recognize synrift geometries. As in the north cross-section (Fig. 5), large basement-involved wedges produce an intense thin-skinned deformation. 2. El Cholar anticline, 3. Manzano anticline, 5. Tres Chorros anticline, 7. Cerro Pitrén anticline, 8. Naunauco syncline, 12. Taquimilán anticline, 16. Maitenes anticline, 15. Pampa Tilhué anticline, 18. San Eduardo anticline. Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 261

FIG. 10. a. Photograph showing the forelimb of the Manzano anticline; b. Photograph of the gently-dipping Vaca Muerta and Muli- chinco formations in the backlimb of the El Cholar anticline; c. Syncline involving the Tordillo Formation near Tres Chorros (see location in figure 3). detached from the underlying evaporites of the The subvolcanic rocks of the Naunauco Group crop Auquilco Formation. out in the core of the Naunauco syncline, a broad fold The Tres Chorros anticline is a tight, doubly- developed to the southwest of Taquimilán (Fig. 3). vergent, N-S fold, cored by the Vaca Muerta Formation The east-vergent Taquimilán anticline is well exposed (Fig. 3). The eastern limb of this anticline is steeply- with a wavelength of approximately 6 km, although dipping, and locally beds become overturned defining with low relief, between the Cerro Naunauco and a dominant eastward movement. At the western limb, the Route 40. The western limb dip of this anticline the Mulichinco Formation is partially thrusted over is 23º-41º W while the eastern limb dip is 68°-72° E the Agrio Formation along the Tres Chorros backthrust (Fig 11a). The whole Agrio Formation is involved in (Fig. 3). Thus, the Tres Chorros anticline represents the Taquimilán anticline, but in some creeks near the a pop-up structure bounded by two opposite-dipping route the Mulichinco Formation is exposed (Fig. 11b). thrusts (Fig. 9). This anticline corresponds to a second order structure In cross-section a-a’, the Cerro Pitrén anticline interpreted as a fault-propagation fold with a is considered a second order east-vergent fold detachment level in the Auquilco Formation (Fig. 9). with subordinate backthrusting in its backlimb, The west-vergent Maitenes anticline forms a involving the Mendoza Group (Fig. 9). Between prominent hill located east of route 40, with intensely the Tres Chorros and Cerro Pitrén anticlines, there deformed shales of the Vaca Muerta Formation in are smaller folds that affect the Agrio, Huitrín and its core and the Mulichinco Formation in its flanks Rayoso formations (Fig. 7). In this zone, there are (Figs. 11a and 12a). The backlimb dip of this anticline two synclines and a tight west-vergent anticline is 20°-30º E and the forelimb dip is 70º-75º W-SW, whose backlimb and forelimb dip 52º E and 73º W although locally it becomes overturned. West of the respectively. These structures have been interpreted Maitenes anticline, a tight syncline involving the Agrio as third order folds with a detachment level in the Formation was identified (Figs. 11a and 12b). In the Pilmatué Member (Fig. 9). western flank of this syncline, the Maitenes backthrust 262 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina

FIG. 11.a. Detailed map of the structures between Taquimilán and Cerro Curacó (see location in figure 3); b. Southward view of the Taquimilán anticline. juxtaposes east dipping beds of the Avilé Member has a gently-dipping backlimb (16º-23º E) and a with top of subhorizontal layers of the Agua de la steeply-dipping (50º-70º W) forelimb (Figs. 9 and Mula Member (Fig. 12c). The average orientation of 12d). The Río Neuquén anticline lay between the the fault plane is 049º/42º (dip direction/dip), with Maitenes and San Eduardo second-order anticlines, striae pitching 72° N. The Maitenes anticline has been few km to the south of the cross-section b-b’ (Figs. 3 interpreted as the result of two deformation phases, and 11a). This fold also has a west vergence, however in which the first one corresponds to a fault-bend given its smaller wavelength and the Agrio Formation fold with an upper decollement on the Agua de la involvement in the deformation it can be considered Mula Member, whose westward movement produced as a third order anticline. the Maitenes backthrust (Fig. 9). Subsequently, a breakthrough fault cut the forelimb increasing its 4. Kinematic evolution of complex thin-skinned dip and the structural relief of the fold. structures The west-vergent San Eduardo anticline locates to the east of the Neuquén River, exposing the Along the northern segment of the Agrio FTB Mulichinco Formation, and it has a wavelength of there are complex thin-skinned structures. The Jurassic approximately 5 km (Fig. 11a). This N-S anticline evaporites of the Auquilco Formation constitute the Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 263

FIG. 12. a. Schematic interpretation of the Maitenes anticline over a 3D Google Earth image; b. Detailed map of third-order structures affecting the Agrio Formation to the west of the Cerro de Los Maitenes; c. Photography of the Maitenes backthrust showing the measurements on the fault plane; d. Photos of the forelimb of the west-vergent San Eduardo anticline. main detachment into the sedimentary sequence, which rocks for the tectonic development of the fold and allows to explain the thin-skinned folds involving thrust belts (Zapata et al., 1999; Zapata and Folguera, the Tordillo Formation in the area of Tres Chorros 2005; Zamora Valcarce et al., 2006; Sánchez et al., (Figs. 3 and 10c). Structural studies in surrounding 2014, 2015; Turienzo et al., 2014, 2018). Secondary areas highlight the importance of these evaporitic detachments located in the Vaca Muerta, Agrio and 264 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina

Huitrín formations favour the formation of several to form a tighter anticline (Suppe and Medwedeff orders of fault-related folds. 1990). In our model, the breakthrough thrust is In our study region, an example of complex along the axial surface of the Tilhué Este anticline, thin-skinned deformation occurs at the Loma Tilhué, which moved passively the Tilhué Oeste anticline where three second-order anticlines are exposed in a and brings to the surface the shales of the Vaca relatively small area (Figs. 3 and 8a). The observed Muerta Formation (Fig. 13a, contractional stage 2). structural configuration results from the combination Finally, a new west-dipping thrust developed in of fault propagation folds with variable vergence that normal sequence toward the foreland forms the involve the entire Mendoza Group (Fig. 5). Pampa Tilhué anticline, folding the previous structures To analyse the kinematic evolution of these (Fig. 13a, contractional stage 3). The subsequent structures, we built a forward model using the formation of the basement-involved Las Yeseras Fault-Fold-Forward software (Allmendinger, 2012). thrust in depth, generated gentle westward tilting of This program allows to reconstruct the structures as the thin-skinned structures. Thus, the final geometry trishear fault-propagation folds and to calculate the of the modelled structures is comparable with the shortening at each deformation stage. This type of exposed structures in the Loma Tilhué along the fault-related fold depends on two variables: the apical cross-section a-a’ (Fig. 13b). angle of the trishear zone and the propagation-to-slip ratio (p/s). In trishear zones with small apical angles, 5. Discussion intense strain is concentrated in a narrow wedge of rock, whereas broader angles result in more diffuse, 5.1. Structural models and variation of tectonic less intense strain (Allmendinger, 1998). On the shortenings other hand, the p/s ratio determines how rapidly the tip line propagates relative to the slip on the Two main modes of deformation were commonly fault itself (Allmendinger, 1998). Low values of p/s used in this region in order to explain the Andean result in pronounced forelimb thickening and tight mountain building: (i) positive inversion of previous folding in the trishear zone, whereas p/s>1 results normal faults (Vergani et al., 1995; Booth and in less thickening and more open folding (Hardy Coward, 1996; Ramos, 1998; Zapata et al., 1999; and Ford, 1997). After testing with different values Zapata and Folguera, 2005; Zamora Valcarce, 2007; for these parameters, we found that the geometry Rojas Vera et al., 2015) and (ii) generation of new and kinematics of the folds in Loma Tilhué can thrust systems (Minniti et al., 1986; Nocioni, 1996; be adequately explained using a trishear apical Kozlowski et al., 1998; Turienzo et al., 2014, 2018; angle of 60° and a p/s ratio of 2. Before starting the Sánchez et al., 2015). Despite both mechanisms model, we scaled the stratigraphical thickness of probably coexisted during the Andean deformation, the sedimentary units involved in the deformation their relative importance and their contribution to (Fig. 13a, pre-contractional stage). the observed contraction along the fold and thrust The beginning of the thin-skinned deformation belts is still a matter of debate (i.e., Mescua and in this area is temporarily related to the Giambiagi, 2012; Dimieri and Turienzo, 2012; insertion of basement-involved wedges onto the Turienzo et al., 2018). Auquilco Formation in the western area (Fig. 5). Interpretation of poor-quality seismic lines Displacement transferred to the east by these in the study area does not show clear evidence thick-skinned structures creates two opposite- of a significant inversion processes. Comparable dipping thrusts in the sedimentary cover that conclusions arise from subsurface interpretations form the Tilhué Oeste and Tilhué Este anticlines (Sánchez et al., 2015; Turienzo et al., 2018) and (Fig. 13a, contractional stage 1). Unfortunately, field observations (Sagripanti et al., 2014) in the there is no evidence for distinguishing which of Chos Malal FTB, where the Late Triassic-Early these folds formed first, or whether they formed Jurassic normal faults were only partially inverted simultaneously, however their combination constitutes and they preserve their extensional character. Many a pop-up structure. As deformation progresses, regional cross-sections have been made along the a breakthrough fault can develop along the fold Agrio and Chos Malal fold and thrust belts in the last when the fault tip encounters layers that are unable three decades, analysing the structural style of the Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 265

FIG. 13. a. Forward modelling constructed with the Fault Fold Forward software (Allmendinger, 2012) for explaining the sequential evolution of the complex structures at the Loma Tilhué, in cross-section a-a’. Initial Stage: Configuration of the Mesozoic units prior to the deformation; Stage 1: Development of two opposite-vergence fault-propagation anticlines creating a pop-up structure; Stage 2: Continued contraction forms an out of sequence breakthrough fault that cuts the Tilhué Este anticline; Stage 3: Formation in sequence of the Pampa Tilhué anticline, folding the previous structures to their final configuration; b. Comparison between the fault-fold forward model and the natural structures along this sector of the structural cross-section a-a’. 266 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina deformed regions of the Neuquén Basin (Fig. 14). south, as revealed by the progressive younging of the Most of the interpretations coincide in that the rock sequence exposed along the Rahueco anticline tectonic shortenings in the Andes of Neuquén (Fig. 3). Calculated shortenings show a small variation decrease toward the south, from the Cordillera del from 11.2 km (18%) in cross section a-a’ to 10.9 km Viento (~37° S) to the Cerro Mocho (~38° S). In (17.3%) in cross-section b-b’ (Fig. 14). These values addition, the calculated magnitudes of contraction are are comparable with the 18% contraction estimated smaller in cross-sections considering the reactivation by Minniti et al. (1986) and they are slightly higher of normal faults as the main mechanism for the fold than the 9.3 km (14%) calculated by Zamora and thrust belt construction (Table 1). Valcarce (2007) in the Cerro Naunauco area. Bigger The north to south decrease in shortening in discrepancies exist with the shortenings derived the Chos Malal FTB coincides with the southward from the regional cross-sections of Nocioni (1996), plunging of the Cordillera del Viento, which suggests where 43 km (34%) of contraction were obtained this major thick-skinned structure strongly controls considering the stacking of many basement-involved the deformation along the FTB (Sánchez et al., sheets and very low angle thrusts. On the other hand, 2015). In the northern Agrio FTB, the thick-skinned Rojas Vera et al. (2015) interpreted along the Cerro structures at the western sector also plunges to the Naunauco a regional cross-section dominated by

FIG. 14. Location of previous structural cross-sections along the Chos Malal and Agrio FTB showing their respective shortenings (from Minniti et al., 1986; Nocioni, 1996; Booth and Coward, 1996; Zapata et al., 1999; Zamora Valcarce, 2007; Sánchez et al., 2015; Rojas Vera et al., 2015 and Turienzo et al., 2018). The differences in the shortenings respond to latitudinal variations and the assumed structural model (see text (section 5 and table 1) for detailed explanations). Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 267

TABLE 1. COMPARISON OF TECTONIC SHORTENING (%) AT DISTINCT LATITUDES AND FOR DIFFERENT STRUCTURAL MODELS.

Shortening (%) by tectonic model Latitude (from North to South) Andean thrusts system Inversion normal faults Cordillera del Viento (37°10’S) 26.3-29.7 (Sánchez et al., 2015) 7.5 (Rojas Vera et al., 2015) 16.5-18 (Turienzo et al., (2018) 20 (Booth and Coward, 1996) 34 (Nocioni, 1996)

Cerro Caicayén (37°27’S) 18 (Lebinson et al., this paper) - 34 (Nocioni, 1996)

Cerro Naunauco (37°33’S) 17.3 (Lebinson et al., this paper) 8.4 (Rojas Vera et al., 2015) 18 (Minniti et al., 1986) 14 (Zamora Valcarce, 2007)

Cerro Mocho (38°S) - 7.4 (Rojas Vera et al., 2015) 8-14 (Zamora Valcarce, 2007) inverted normal faults, calculating a shortening of several E-W basaltic dykes with 40Ar/39Ar ages of only 9.6 km (8.4%). It should be noted that since 101.9±0.69 Ma and 91.97±4.06 Ma that cross-cut the sections interpreted by the different authors do the folds, suggesting a Cretaceous E-W compressive not have the same lengths, the percentage shortening event prior to ~100 Ma (Zamora Valcarce et al., should be compared in order to analyse variations 2006, 2009). These values are unique in the whole in contraction. region and it should not be ruled out that a possible In the studied segment of the Agrio FTB our enrichment of argon has resulted in older ages estimated shortenings result dominantly from the (Llambías et al., 2011). restoration of the sedimentary cover, which contains Cobbold and Rossello (2003) observed in outcrops six thin-skinned fault-related folds constrained by and in seismic lines an angular unconformity between field data (Figs. 5 and 9). The lack of evidence the Huitrín and Rayoso formations, approximately of relevant inversion tectonics, together with the 25 km to the southeast of the study area. Thus, they mechanical difficulty of high-angle normal faults to proposed the beginning of Andean compression during accommodate considerable contractions, led us to the upper Early Cretaceous (Aptian-Albian). Other consider an alternative interpretation. We propose a studies consider that the red beds of the Neuquén regional structural model in which major basement- Group, which lie unconformably over the Rayoso involved thrusts form first order wedges and transfer Formation, are synorogenic deposits accumulated in a displacement to the cover, producing second order foreland basin related to Late Cretaceous deformation fault-related folds. Moreover, some second order (Ramos, 1981; Ramos and Folguera, 2005; Zamora structures are interpreted as fault-bend anticlines Valcarce et al., 2009; Tunik et al., 2010). In addition, with an upper decollement in shallow stratigraphic Cobbold and Rossello (2003) argued that folding levels, whose displacement produces minor-scale was active from the Late Cretaceous and continued structures (third and fourth order). until the Eocene based on the angular unconformity between Cayanta Formation (39±9.1 Ma 40K/40Ar, 5.2. Age of deformation and structural evolution Llambías and Rapela, 1989) and the Bajada del Agrio Group in the Colipilli region. Through the study The analysis of the structures in the study area, in of apatite fission tracks, Rojas Vera et al. (2015) conjunction with the existing ages of the igneous rocks proposed that the Cretaceous deformation extends exposed in the region, allows the reconstruction of a to the Paleocene, and perhaps until the Eocene. possible sequence of deformation for the northernmost Our kinematic model, based on the cross-section segment of the Agrio FTB (Fig. 15). a-a’, indicates that the initial deformation occurred In the region of Cerro Mocho, approximately during the Late Cretaceous-Eocene stage with the 75 km to the south of Chos Malal city, there are formation of the Rahueco basement-involved wedge 268 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina

FIG. 15. Proposed kinematic evolution of the tectonic structures along a-a’ structural cross-section. a. Stage prior to the Andean contraction (before the Late Cretaceous); b. The Rahueco anticline, formed during the Late Cretaceous-Eocene compressive phase that is related to a basement-involved thrust that moves eastward along the Auquilco Formation generating the thin- skinned Cerro Pitrén, Taquimilán Centro and Truquico anticlines; c. The Lower Rahueco basement-involved thrust generates the structures at Loma Tilhué during the Middle-Late Miocene, simultaneously with synorogenic sedimentation of the Chos Malal Formation; d. Generation of the thick-skinned Neuquén thrust that partially cuts an inverted normal fault forming the Pampa Tilhué anticline. Generalized rotation of the easternmost structures occurs over the hangingwall of the Las Yeseras thrust, probably in the Late Miocene. Lebinson et al. / Andean Geology 45 (2): 249-273, 2018 269

(Fig. 15 stage 1). As this first-order structure moves balanced cross-sections that allow the characterization eastward along the evaporites of the Auquilco of the structural style of the northern Agrio FTB. Formation it generates the Cerro Pitrén anticline The structures recognized in this region include (second-order structure), which in turn transmits first-order basement-involved anticlines, with large deformation to shallower levels forming third-order fold wavelengths, and second-order thin-skinned structures as the Taquimilán Centro and Truquico anticlines with shorter wavelengths. Moreover, anticlines. A deformation model that links thick- minor-scale structures (third and fourth order), that skinned structures with thin-skinned fault-related involve successively shallower stratigraphic units folds of different order was proposed and documented were recognised. Forward modelling of second- in the Chos Malal FTB (Kozlowski et al., 1998; order folds allows to understand the kinematics of Sánchez et al., 2014, 2015; Turienzo et al., 2014, complex thin-skinned structures as the Loma Tilhué 2018). The Eocene igneous rocks that form the anticlines that formed by the combination of a pop- Cerro Caicayén intrude Jurassic strata using the up and imbricate thrust-related-folds. Because of backthrusts in the forelimb of the Rahueco anticline the poor quality of the seismic data, it was difficult as feed channels, suggesting that the deformation of to recognize the precise geometry of the structures the area predated intrusion or were contemporaneous in depth. However, in the northernmost seismic (Lebinson et al., 2015b). section, synrift geometries related with normal faults Synorogenic accumulation of the Puesto Burgos, with slight positive inversion were inferred. The Rincón Bayo and Chos Malal formations (Cervera restoration of the structural cross-sections shows and Leanza, 2009), together with the Tralalhué tectonic shortenings ranging from 11.2 km (18%) Conglomerate (Ramos, 1998), evidence a Middle- to 10.9 km (17.3%), suggesting a slight southward Late Miocene contractional event in the Agrio FTB. decrease of contraction, consistent with previous Zamora Valcarce et al. (2009) correlate the deposits of interpretations along the Neuquén Andes. Puesto Burgos and Rincón Bayo formations with two A structural model that explains the development deformation events, at 11 Ma and 6 Ma, interpreted of the northern Agrio FTB is proposed. This model from apatite fission track analysis in the Dorso de considers a close temporal and spatial relationship los Chihuidos. From the sequential reconstruction, a between the thick and the thin skinned structures second basement involved thrust formed during the developed during the Andean orogeny. Throughout Middle-Late Miocene compression, in which both the study area, major thrusts produce first order reactivated previous structures and created the fault- basement-involved wedges that move forward on a related folds in the Loma Tilhué (Fig. 15 stage 2). decollement surface in the Late Jurassic evaporites Sedimentation of the Chos Malal Formation at this of the Auquilco Formation. These transferred stage took place in a small piggy back basin restricted displacements give rise to second order fault- to the Truquico syncline (Cervera and Leanza, propagation and fault-bend folds in the overlying 2009). Folding and meter scale structures within rocks. The proposed structural analysis suggests a this unit evidence that deformation continued after model of thick-skinned deformation, characterized by its sedimentation. As deformation progressed, a third newly developed Andean thrust systems as the main basement-involved thrust cut a partially inverted normal mechanism producing contraction in this fold and fault, transmitting displacement to the cover and the thrust belt. By means of an integration of the studied consequent creation of the Pampa Tilhué anticline structures with the existing geochronological data, (Fig. 15 stage 3). Finally, the basement-involved Las a kinematic evolution that explains the structural Yeseras thrust forms toward the foreland, out of the development of the northern Agrio FTB in two main study area, creating the regional slope observed in compressive phases (Late Cretaceous-Eocene and the easternmost portion of the cross-sections and in the Middle-Late Miocene) is proposed. the seismic lines. Acknowledgments 6. Conclusions This work was financed by CONICET PIP 0583, CONICET P-UE47CO, FONCYT PICT 2015-0419 Based on detailed field mapping and of 2D y SECYTUNS PGI 24/h136. We are grateful to the seismic sections interpretation we constructed two Departmento de Geología of the Universidad Nacional 270 The structure of the northern Agrio fold and thrust belt (37°30’S), Neuquén Basin, Argentina del Sur and INGEOSUR-CONICET for logistical and Danieli, J.C.; Vallés, J.M.; editors). Asociación financial support. We also acknowledge the Subsecretaría de Geológica Argentina: 49-62. Buenos Aires. Energía, Minería e Hidrocarburos of the Neuquén Province Digregorio, J.H.; Uliana, M. 1980. Cuenca Neuquina. for providing the seismic lines and well information, and In Simposio de Geología Regional Argentina, No. 2 the Municipalidad of Chos Malal for supporting the field (Turner, J.C.; editors): 985-1032. Córdoba. work. We greatly appreciate the comments and suggestions Dimieri, L.; Turienzo, M. 2012. Comment on: “Fault by reviewer J. Skármeta and Editor W. Vivallo, which inversion vs. new thrust generation: A case study in the considerably improved this paper. 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Manuscript received: March 16, 2017; revised/accepted: November 7, 2017; available online: January 31, 2018. Editorial handling: Jorge Skármeta.